1. Field of the Invention
The present invention relates to a linear electrodynamic-type motor for compressing a fluid circulating in a cryocooler, notably using a Stirling cycle. In addition, the present invention relates to a Stirling cycle cryocooler including such a motor. Furthermore, the present invention relates to a method for compressing a fluid circulating in such a cryocooler.
The present invention finds particular application in the field of alternating cycle cryogenic machines, Stirling machines or pulsed gas tubes, implementing linear electrodynamic reciprocating motors, in particular cryogenic machines intended to be placed on board spacecraft such as Earth observation satellites. In this application, a linear electrodynamic motor is used as a compressor for compressing a fluid such as helium, whose expansion causes cooling.
2. Related Art
A prior art linear electrodynamic motor generally includes two translationally movable induction coils, an AC power-supply circuit for the induction coils and two pistons connected respectively to the two induction coils. Each piston is mounted on a bearing that develops an axial elastic return force proportional to the displacement of the piston. Under the effect of cyclic magnetic forces, the induction coils drive the pistons with a reciprocating linear motion. The pistons form translationally movable masses, thereby compressing the fluid.
In order to maximize the amplitude of the displacement of each piston, and therefore the compression of the fluid, the motor is controlled so that the pistons operate at or near their mechanical resonance frequency. In addition, to the same end, the motor is designed for reducing the damping forces of the pistons, especially friction.
However, this reduction in damping forces makes the motor sensitive to vibrations and shocks, which can cause excessive oscillations of the pistons producing internal shocks between each piston and the fixed parts of the motor, which reduces the performance and/or service life of the motor. But in the field of spacecraft, launching induces many shocks and vibrations.
One solution is to equip the motor with secondary circuits connecting the respective terminals of the induction coils so as to short-circuit them. Thus, when shocks or vibrations displace the pistons, the change in magnetic flux in the coils induces, according to Lenz's law, a counter-electromotive force therein, which generates a magnetic force opposing the displacement of the pistons and damping their displacement.
However, the magnetic force generated is insufficient to damp major vibrations, especially since it is determined by the size of the induction coils. Furthermore, the inductance of an induction coil leads to a phase shift that causes a delay in the generation of the magnetic force damping the pistons which are therefore still displaced with an excessive amplitude.